A covercoated thermal transfer sheet for transferring an image to a ceramic substrate at ambient temperature.
U.S. Pat. No. 5,069,954 of Donald Cole et al. describes a transfer assembly for use with automatic offset application equipment. The patent claims: xe2x80x9c1. A ceramic transfer for use with automatic offset application equipment wherein the transfer is applied to ware by means of a printing pad, the transfer comprising, in sequence, a backing paper, release layer, design layer and covercoat layer, in which the covercoat layer is formed from a heat-activatable adhesive and extends over an area greater than that of the design layer so as to cover the design layer and provide a marginal portion of the covercoat layer which is in direct contact with the release layer, and the design layer is formed from a printing ink the medium of which has heat-activatable adhesive properties, the covercoat layer being such that when its adhesive is heat-activated, the marginal portion thereof has a lower coefficient of adhesion to the release layer than for the printing pad and the covercoat has a lower coefficient of adhesion for the printing pad than the marginal portion of the covercoat and the design layer have for the ware so that the design layer can be separated from the release layer and transferred to the ware by the pad.xe2x80x9d
It is disclosed in the Cole et al. patent that the prior art systems must be operated within certain specified temperature ranges. Thus, e.g., in the last paragraph of column 1 of U.S. Pat. No. 5,069,954, it is disclosed that: xe2x80x9cWe have found . . . it is difficult . . . to achieve consistent clean transfer without the use of an overlying adhesive layer to aid separation from the backing paper. Furthermore, the development of adhesive properties only within specified temperature ranges, as taught by U.S. Pat. No. 3,967,021, renders the successful operation of the process vulnerable to changers in ambient temperature and/or time delays . . . . xe2x80x9d
The process of the Cole et al. patent also requires the use of xe2x80x9c. . . specified temperature ranges . . . . xe2x80x9d Thus, as is disclosed at lines 37 to 44 of column 5 of this patent, xe2x80x9cTypically, the pad is maintained at a temperature of 100 degrees-130 degrees C. and a heated vacuum platen or base plate is maintained at a temperature of 70 degrees-100 degrees C. . . . . xe2x80x9d When temperatures in excess of those specified are used in the Cole et al. process, distortion of the image being transferred often occurs.
However, the time at which the transfer paper is heated in the Cole et al. process is critical. This dwell time is described at lines 42 to 44 of column 5 of such patent, wherein it is taught that: xe2x80x9cTypically, the pad resides on the ware for approximately half a second to ensure adequate adhesion of the design layer to the ware.xe2x80x9d Dwell times substantially longer than about half a second often causes distortion of the image being transferred when the Cole et al. process is used.
In the ceramic transfer assembly described in the Cole et al. patent, an image or design layer is printed upon a wax release layer (see, e.g., lines 63 to 66 of column 3 of the patent). Such a wax release layer is not optimally suited for receiving printed images due to its lower surface energy; and the images thus received often lack sufficient adhesion and often have poor image quality. Because of its poor adhesive qualities, the image printed on the release layer requires a covercoat layer in the Cole et al. assembly. It is expensive, difficult, and cumbersome to covercoat an image once it has been printed onto the wax release layer.
Furthermore, the process of U.S. Pat. No. 5,069,954 is limited in that it requires the use of pad transfer equipment and heat transfer paper and, thus, cannot be used, e.g., at ambient temperature.
It is an object of this invention to provide a process for transferring an image to a ceramic substrate which is readily adapted to use with substrates that need not have flat surfaces.
It is another object of this invention to provide a process for transferring an image to a ceramic substrate that need not be operated at elevated temperatures.
It is yet another object of this invention to provide a process for transferring an image to a ceramic substrate that need not be operated within a specified period of time.
It is yet another object of this invention to provide a process and an assembly for transferring an image to a ceramic substrate that provides better adhesion and print quality than does the process of U.S. Pat. No. 5,069,954.
It is yet another object of this invention to provide a covercoated transfer sheet for transferring images to a ceramic substrate which sheet is readily adapted for use in the process of the invention.
In accordance with this invention, there is provided a covercoated transfer sheet for transferring images to a ceramic substrate, wherein said covercoated transfer sheet is comprised of a flat, flexible substrate and a transferable covercoat releaseably bound to said flat, flexible substrate, wherein, when said transferable covercoat is printed with an image to form an imaged covercoat, said image has a higher adhesion to said covercoat than said covercoat has to said flexible substrate, said imaged covercoat has an elongation to break of at least about 1 percent, and said imaged covercoat can be separated from said flexible substrate with a peel force of less than about 30 grams per centimeter.